An alien divalent ion reveals a major role for Ca²⁺ buffering in controlling slow transmitter release

J Neurosci. 2014 Sep 17;34(38):12622-35. doi: 10.1523/JNEUROSCI.1990-14.2014.


Ca(2+)-dependent transmitter release occurs in a fast and in a slow phase, but the differential roles of Ca(2+) buffers and Ca(2+) sensors in shaping release kinetics are still controversial. Replacing extracellular Ca(2+) by Sr(2+) causes decreased fast release but enhanced slow release at many synapses. Here, we established presynaptic Sr(2+) uncaging and made quantitative Sr(2+)- and Ca(2+)-imaging experiments at the mouse calyx of Held synapse, to reveal the interplay between Ca(2+) sensors and Ca(2+) buffers in the control of fast and slow release. We show that Sr(2+) activates the fast, Synaptotagmin-2 (Syt2) sensor for vesicle fusion with sixfold lower affinity but unchanged high cooperativity. Surprisingly, Sr(2+) also activates the slow sensor that remains in Syt2 knock-out synapses with a lower efficiency, and Sr(2+) was less efficient than Ca(2+) in the limit of low concentrations in wild-type synapses. Quantitative imaging experiments show that the buffering capacity of the nerve terminal is markedly lower for Sr(2+) than for Ca(2+) (~5-fold). This, together with an enhanced Sr(2+) permeation through presynaptic Ca(2+) channels (~2-fold), admits a drastically higher spatially averaged Sr(2+) transient compared with Ca(2+). Together, despite the lower affinity of Sr(2+) at the fast and slow sensors, the massively higher amplitudes of spatially averaged Sr(2+) transients explain the enhanced late release. This also allows us to conclude that Ca(2+) buffering normally controls late release and prevents the activation of the fast release sensor by residual Ca(2+).

Keywords: calcium buffering capacity; calcium sensor; endogenous fixed buffer; slow release sensor; strontium; synaptotagmin.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Calcium / metabolism*
  • Chelating Agents / pharmacology
  • Membrane Potentials / drug effects
  • Membrane Potentials / physiology
  • Mice
  • Mice, Knockout
  • Nerve Endings / drug effects
  • Nerve Endings / metabolism
  • Nerve Endings / physiology*
  • Optical Imaging
  • Strontium / metabolism*
  • Synaptic Transmission / drug effects
  • Synaptic Transmission / physiology*
  • Synaptotagmin II / genetics
  • Synaptotagmin II / metabolism


  • Chelating Agents
  • Synaptotagmin II
  • Calcium
  • Strontium